This invention relates to acoustic surface wave devices. More particularly, it is concerned with acoustic surface wave devices employed as filters.
Acoustic surface wave devices employing piezoelectric materials having suitable properties for propagating surface waves and having transducers for launching and receiving acoustic surface waves in the material are well-known. Typically, the transducers are arrays of interleaved conductive electrodes deposited on the material. In response to electrical signals an input or transmitting transducer launches acoustic surface waves along a predetermined path on the surface of the material. An output or receiving transducer detects the acoustic surface waves and generates electrical signals in response thereto. Typically, acoustic surface wave devices have been employed as delay lines and as filters. Because of the frequency response which can be obtained in an acoustic surface wave device by suitably designing the configuration of the transducer electrodes, particularly desirable bandpass characteristics can be achieved such as that required of an intermediate frequency filter for use in television receivers.
In the development of acoustic surface wave devices for use as filters various problems have been encountered. Several secondary effects are present which tend to degrade the performance of the device. Various techniques have been employed to compensate for or avoid certain of these secondary effects.
One undesirable secondary effect is known as wavefront distortion. In order to obtain the desired frequency response for certain types of filters the electrodes of the input transducer extending in opposite directions from the two bus bars are arranged to overlap. With varying overlap the number of metal electrodes traversed by a surface wave moving along its path of propagation varies across the span of the transducer aperture. Since the velocity of acoustic surface waves is affected by travelling under a metallized surface, the result is wavefront distortion. It has been found that this secondary effect can be avoided by the use of so-called "dummy" or inactive electrodes which extend toward each active electrode from the opposite bus bar so as to provide an overall generally rectangular configuration of the transducer. Thus all acoustic surface waves generated within the overlap region of the transducer aperture traverse essentially the same amount of metallized surface as they pass along the propagation path through the transducer.
Another secondary effect is acoustic reflections caused by impedance discontinuities in the propagating medium. This problem is corrected by the use of dual element electrodes in place of single element electrodes. With single element electrodes the electrodes are generally one-fourth of the principal wavelength wide and adjacent electrodes are generally separated by one-fourth of a wavelength. With the two element electrodes each element is one-eighth of a wavelength wide and adjacent elements are separated by one-eighth of a wavelength. The double element electrode configuration causes undesirable acoustic reflections to cancel each other. This technique is well-known and widely used to suppress what is known as triple transit echoes.
Another secondary effect problem is caused by reflections occurring at the edges of the electrodes with either single element or two element electrodes. Although double element electrode structures are efficient in suppressing reflections at the center frequency of the device, this action degrades gradually outward from the center frequency. In many types of acoustic surface wave devices the electrode structure is weighted as to amplitude and phase; that is, the length of the electrodes is varied to vary the overlap and the spacing between the electrodes is varied to produce phase weighting. The problem of reflections from electrode edges may be exaggerated in devices of this type. Although individual edge reflections are small, they can add in phase to significant values to become noticeable spurious signals.
One technique for reducing the problem of electrode edge reflections is disclosed in U.S. Pat. No. 4,023,124 which issued to D. W. Parker et al on May 10, 1977. In this patent the input transducer electrode structure disclosed employs single element electrodes in the overlap region and double element electrode structure for the inactive electrodes and for the connections to the active electrodes. As explained in the patent the primary purpose of this configuration is to permit the use of wider active electrodes in the overlap region and thus reduce problems in the fabrication of the thinner double element active electrodes while at the same time providing a metallized structure which will prevent wavefront distortion. However, this configuration employing single element electrodes in the overlap region fails to correct for problems of electrode interaction distortion which typically are eliminated by the use of double element electrodes.